This invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement in anode material.
Hitherto, as a secondary battery of general use, secondary batteries of the aqueous solution system such as lead battery, nickel/cadmium battery, etc. were the main current. However, while these aqueous solution system secondary batteries are satisfactory to some extent with respect to the cycle characteristic, it cannot be said that they have the characteristics which are satisfactory in connection with the battery weight and/or the energy density.
On the other hand, in recent years, researches and developments of non-aqueous electrolyte secondary batteries using lithium metal or lithium alloy as anode are being extensively carried out. These batteries have excellent characteristics of high energy density, less self-discharge and light weight.
However, when charge/discharge cycle is repeatedly carried out, there is the drawback that lithium is crystal-grown in a dendrite form on the anode in charge and the lithium in the dendrite form reaches the cathode, leading to the internal short. This seriously impedes practical use thereof.
In view of the above, as a non-aqueous electrolyte secondary battery to solve such problems, the spot light of attention has been focused on non-aqueous electrolyte secondary battery using carbon material as the anode. This non-aqueous electrolyte secondary battery utilizes doping/undoping of lithium into portions between carbon layers as anode reaction. Even if the charge/discharge cycle is repeatedly carried out,-precipitation in dendrite form of lithium cannot be observed. As a result, such non-aqueous electrolyte secondary battery exhibits satisfactory charge/discharge cycle characteristic.
Meanwhile, various kinds of carbon materials are enumerated as carbon materials which can be used as anode material of the above-mentioned non-aqueous electrolyte secondary battery. The material which has been first put into practical use as anode material is a non-graphitic carbon, e.g., a graphitizable carbon such as a coke, a non-graphitizable carbon such as a glass like carbon having low crystallinity obtained carrying out heat treatment of organic material at a relatively low temperature. Non-aqueous electrolyte secondary batteries using anode constituted with such non-graphitizable carbon material and the non-aqueous electrolyte including propylene carbonate (PC) as major solvent have been already commercialized.
Further, in recent years, graphite materials of which crystal structure has been developed has been also able to be used. In the case of graphite materials, PC used as the major solvent would be decomposed by graphite materials. This constituted obstruction in use as the anode material. However, by employing ethylene carbonate (EC) having high stability against graphite materials as the major solvent, such a problem is eliminated. Thus, use as the anode material can be made.
Flaky graphite materials can be relatively easily obtained, and has been conventionally widely used as conductive agent for alkaline primary battery. This graphite materials has higher crystallinity and higher true density as compared to the non-graphitizable carbon material. Accordingly, if the anode is constituted by such flaky graphite, high electrode packing density can be obtained. Thus, the energy density of battery is enhanced. From this fact, it can be said that graphite material is greatly expected material as the anode material.
Meanwhile, the material characteristics of graphite material diversely vary in dependency upon the production process thereof, and characteristics as the anode material also vary followed by this.
In order to obtain high reliability non-aqueous electrolyte secondary batteries as the industrial product, graphite materials of fixed (uniform) characteristic must be selected as a matter of course. For this reason, various studies have been conducted with respect to material characteristic values which affect the characteristic as the anode material of the graphite material to be selected.
For example, because graphite material has high crystallinity, crystal structure parameters determined by the X-ray diffraction or the Raman spectrometry, etc. provide a clue for recognition of doping/undoping ability of lithium.
Moreover, true density determines the electrode packing density. Materials having higher true density are permitted to have higher electrode packing density. As described above, in the graphite materials, it is one of merits that the true density is higher than that of the non-graphitizable carbon material.
In addition, particle diameter and/or specific surface area are also effective material characteristic values for selection of materials excellent in the characteristic.
However, when graphite material selected only by the above-mentioned material characteristic values is used as the anode, there are instances where the cycle lifetime is extremely shorter than that of the battery using non-graphitizable carbon material as the anode. For this reason, further study of the material characteristics are being required.
An object of this invention is to provide a non-aqueous electrolyte secondary battery caused to have higher electrode packing density, higher energy density and long cycle lifetime so that high reliability can be obtained.
As a result of the fact that the inventors of this application have energetically conducted studies in order to attain the above-described object, they have obtained the findings that the cycle lifetime of the battery is caused to be shorter in the case where particularly bulky flaky graphite having high degree of flatness is used, and if graphite having high bulk density and relatively low degree of flatness is selected even in the case where graphite material is used as the anode, elongation of the cycle lifetime can be made.
Moreover, the inventors have obtained the finding that crushed graphite powder or graphite powder having a particular specific surface area is used, whereby the cycle lifetime can be further prolonged.
Further, the inventors have obtained the finding that the graphitized molding material obtained by allowing the carbon molding material to undergo heat treatment to graphitize it is crushed, whereby graphite powder having high bulk density and low degree of flatness can be easily obtained.
Furthermore, the inventors have obtained the finding that graphite powder having a specific grain (particle) size distribution is used, whereby initial failure can be reduced and elevation of the battery temperature in the abnormal state such as overcharge, etc. can be suppressed.
In addition, the inventors have obtained the finding that graphite powder having average value of particle breaking strength is used, whereby improvement in the drain capability can be made.
This invention has been completed on the basis of such findings. A non-aqueous electrolyte secondary battery including an anode consisting of carbon material in which doping/undoping of lithium is permitted, a cathode and a non-aqueous electrolyte in which electrolyte is dissolved in a non-aqueous solvent is characterized in that carbon material constituting the anode is crushed graphite material in which true density is 2.1 g/cm3 or more, and bulk density is 0.4 g/cm or more which is obtained by crushing a graphite raw material.
The graphite material is required to be powder in which average value of shape parameters x indicated by the following expression is 125 or less.
x=(L/T)xc2x7(W/T)
x: Shape parameter
T: Thickness of the portion in which thickness is the thinnest of the powder
L: Length in length axis direction of the powder
W: Length in a direction perpendicular to the length axis of the powder.
Moreover, it is desirable that graphite material is powder having specific surface area of 9 m2/g or less.
Further, it is preferable that graphite material used has, in the grain size distribution determined by the laser diffraction method, the accumulated 10% particle diameter of 3 xcexcm or more, the accumulated 50% particle diameter of 10 xcexcm or more, and the accumulated 90% particle diameter of 70 xcexcm or less.
Such graphite material powder can be obtained, e.g., by crushing the graphitized molding material obtained by allowing the carbon molding material to undergo heat treatment to graphitize it.
It is preferable that the average value of particle breaking strength of the graphite material is 58.84 N/mm2 (6.0 kgf/mm2) or more.
On the other hand, as the cathode, there is enumerated lithium transition metal compound oxide expressed by general expression LiMO2 (In this formula, M represents at least one of Co, Ni, Mn, Fe, Al, V and Ti), and as the non-aqueous electrolyte, there is enumerated the solution in which lithium salt is dissolved into a mixed solvent of cyclic carbonic ester and chain carbonic ester.
In this invention, as described above, there is used a graphite material of true density of 2.1 g/cm3 or more and bulk density of 0.4 g/cm3 or more as the anode material of the non-aqueous electrolyte secondary battery.
Since graphite material has high true density, when the anode is constituted by such graphite material, the electrode packing density is enhanced, so the energy density of the battery is improved.
Moreover when graphite material particularly having bulk density of 0.4 g/cm3 or more of graphite materials is used, for the reason why graphite material having great bulk density as described above can be relatively uniformly dispersed into anode depolarizing mix layer, or the like, the electrode structure becomes satisfactory. Thus, the cycle characteristic can be improved.
Similarly, when graphite material having low degree of flatness in which the bulk density is 0.4 g/cm3 or more and average shape parameter xave is 125 or less is used, the electrode structure becomes more satisfactory. Thus, a longer cycle lifetime can be attained.
In order to obtain graphite material of this invention, it is preferable to employ a method of carrying out heat treatment for graphitization in the state where carbon is caused to be molded body. By crushing such graphitized molding material, graphite material having higher bulk density and small average shape parameter xave can be made up.
Moreover, in the case where graphite powder in which the bulk density and the average shape parameter xave are respectively within the above-mentioned ranges and the specific surface area is 9 m2/g or less is used, there result less fine particle of sub micron attached on the graphite particle, so the bulk density becomes high. Thus, the electrode structure becomes satisfactory, and the cycle characteristic is further improved.
Further, when there is used a graphite powder such that the accumulated 10% particle diameter is 3 xcexcm or more, the accumulated 50% particle diameter is 10 xcexcm or more, and the accumulated 90% particle diameter is 70 xcexcm or less in the grain size distribution determined by the laser diffraction method, a non-aqueous electrolyte secondary battery of high safety and reliability can be obtained. The specific surface area of particles having small grain size becomes large. However, by limiting the content percentage thereof, it is possible to suppress extraordinary heat at the time of overcharge, etc. by particles of great specific surface area. Moreover, by limiting content percentage of particles of higher grain size, it is possible to suppress internal short followed by swelling (expansion) of particles at the time of initial charging. Thus, a practical non-aqueous electrolyte secondary battery having high safety and reliability can be provided.
Namely, in this invention, with respect to graphite material used as anode material of the non-aqueous electrolyte secondary battery, the bulk density, the average shape parameter xave, the specific surface area, the grain size distribution, the method of manufacturing graphite material powder, and the average value of particle breaking strength are limited. Accordingly, it is possible to acquire a non-aqueous electrolyte secondary battery which has high electrode packing density, can obtain anode satisfactory in the electrode structure, has high energy density, exhibits satisfactory cycle characteristic, has longer cycle lifetime, has high safety and reliability, has excellent drain capability, and has high reliability as the industrial product.